Anaximander of Miletus (c.610–c.540 BC)

Greek philosopher of the Milesian School, student of Thales,
and possibly the first person to speculate on the existence of other worlds.
Anaximander held that the fundamental essence of all things is not a particular
substance, like water, but apeiron, or the infinite. It seemed reasonable
to him, given this boundless creative source extending in all directions,
that there might be an indefinite number of worlds existing throughout time,
worlds that "are born and perish within an eternal or ageless infinity."
The pluralism he taught, therefore, was
not of a multitude of planets in space but of an endless temporal succession.

Anaximander also pioneered the notion that the Earth is not flat, suggesting
it was cylindrical and that it floated free, unsupported, at the exact center
of the Universe, with people living on one of the flat ends. As for the
Sun, he said it was as large as the Earth – an audacious theory at
that time.